SWITCHING REGULATORS
Introduction
The switching regulator is increasing in popularity because it offers the advantages of higher
power conversion efficiency and increased design flexibility (multiple output voltages of
different polarities can be generated from a single input voltage). Although most power
supplies used in amateur shacks are of the linear regulator type, an increasing number of
switching power supplies have become available to the amateur. For most amateurs the
switching regulator is still somewhat of a mystery. One might wonder why we even bother
with these power supplies, when the existing linear types work just fine. The primary
advantage of a switching regulator is very high efficiency, a lot less heat and smaller size. To
understand how these black boxes work let’s take a look at a traditional linear regulator at
right. As we see in the diagram, the linear regulator is really nothing more than a variable
resistor. The resistance of the regulator varies in accordance with the load resulting in a
constant output voltage
The primary filter capacitor is placed on the input to the regulator to help filter out the 60
cycle ripple. The linear regulator does an excellent job but not without cost. For example, if
the output voltage is 12 volts and the input voltage is 24 volts then we must drop 12 volts
across the regulator. At output currents of 10 amps this translates into 120 watts (12 volts
times 10 amps) of heat energy that the regulator must dissipate. Is it any wonder why we have
to use those massive heat sinks? As we can see this results in a mere 50% efficiency for the
linear regulator and a lot of wasted power which is normally transformed into heat.
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The time that the switch remains closed during each switch cycle is varied to maintain a
constant output voltage. Notice that the primary filter capacitor is on the output of the
regulator and not the input. As is apparent, the switching regulator is much more efficient
than the linear regulator achieving efficiencies as high as 80% to 95% in some circuits. The
obvious result is smaller heat sinks, less heat and smaller overall size of the power supply.
The previous diagram is really an over simplification of a switching regulator circuit. An
actual switching regulator circuit more closely resembles the circuit below: Now lets take a
look at a very basic switching regulator at right. As we see can see, the switching regulator is
really nothing more than just a simple switch. This switch goes on and off at a fixed rate
usually between 50Khz to 100Khz as set by the circuit.
As we see above the switching regulator appears to have a few more components than a
linear regulator. Diode D1 and Inductor L1 play a very specific role in this circuit and are
found in almost every switching regulator. First, diode D1 has to be a Schottky or other very
fast switching diode. A 1N4001 just won't switch fast enough in this circuit. Inductor L1 must
be a type of core that does not saturate under high currents. Capacitor C1 is normally a low
ESR (Equivalent Series Resistance) type. To understand the action of D1 and L1, let’s look at
what happens when S1 is closed as indicated below:
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As we see above, L1, which tends to oppose the rising current, begins to generate an
electromagnetic field in its core. Notice that diode D1 is reversed biased and is essentially an
open circuit at this point. Now let’s take a look at what happens when S1 opens below:
As we see in this diagram the electromagnetic field that was built up in L1 is now discharging
and generating a current in the reverse polarity. As a result, D1 is now conducting and will
continue until the field in L1 is diminished. This action is similar to the charging and
discharging of capacitor C1. The use of this inductor/diode combination gives us even more
efficiency and augments the filtering of C1. Because of the unique nature of switching
regulators, very special design considerations are required. Because the switching system
operates in the 50 to 100 kHz region and has an almost square waveform, it is rich in
harmonics way up into the HF and even the VHF/UHF region. Special filtering is required,
along with shielding, minimized lead lengths and all sorts of toroidal filters on leads going
outside the case. The switching regulator also has a minimum load requirement, which is
determined by the inductor value. Without the minimum load, the regulator will generate
excessive noise and harmonics and could even damage itself. (This is why it is not a good
idea to turn on a computer switching power supply without some type of load connected.) To
meet this requirement, many designers use a cooling fan and or a minimum load which
switches out when no longer needed. Fortunately, recent switching regulator IC's address
most of these design problems quite well.
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IC 723 – GENERAL PURPOSE REGULATOR
Disadvantages of fixed voltage regulator:
1. Do not have the shot circuit protection
2. Output voltage is not adjustable
These limitations can be overcomes in IC723.
Features of IC723:
1. Unregulated dc supply voltage at the input between 9.5V & 40V
2. Adjustable regulated output voltage between 2 to 3V.
3. Maximum load current of 150 mA (ILmax = 150mA).
4. With the additional transistor used, ILmax upto 10A is obtainable.
5. Positive or Negative supply operation
6. Internal Power dissipation of 800mW.
7. Built in short circuit protection.
8. Very low temperature drift.
9. High ripple rejection.
The simplified functional block diagram can be divided in to 4 blocks.
1. Reference generating block
2. Error Amplifier
3. Series Pass transistor
4. Circuitry to limit the current
1. Reference Generating block:
The temperature compensated Zener diode, constant current source & voltage reference
amplifier together from the reference generating block. The Zener diode is used to generate a
fixed reference voltage internally. Constant current source will make the Zener diode to
operate at affixed point & it is applied to the Non – inverting terminal of error amplifier. The
Unregulated input voltage ±Vcc is applied to the voltage reference amplifier as well as error
amplifier.
2. Error Amplifier: Error amplifier is a high gain differential amplifier with 2 input (inverting
& Non-inverting). The Non-inverting terminal is connected to the internally generated
reference voltage. The Inverting terminal is connected to the full regulated output voltage.
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3. Series Pass Transistor: Q1 is the internal series pass transistor which is driven by the error
amplifier. This transistor actually acts as a variable resistor & regulates the output voltage.
The collector of transistor Q1 is connected to the Un-regulated power supply. The maximum
collector voltage of Q1 is limited to 36Volts. The maximum current which can be supplied by
Q1 is 150mA.
4. Circuitry to limit the current: The internal transistor Q2 is used for current sensing &
limiting. Q2 is normally OFF transistor. It turns ON when the IL exceeds a predetermined
limit.
Low voltage , Low current is capable of supplying load voltage which is equal to or
between 2 to 7Volts.
Vload = 2 to 7V
Iload = 150mA
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IC723 as a LOW voltage LOW current
IC723 as a HIGH voltage HIGH Current
An external transistor Q is added in the circuit for high voltage low current regulator to
improve its current sourcing capacity.
For this circuit the output voltage varies between 7 & 37V.
Transistor Q increase the current sourcing capacity thus IL(MAX) ia greater than 150mA.
The output voltage Vo is given by ,
𝑅1 + 𝑅2
𝑉0 =
𝑉𝑟𝑒𝑓
𝑅2
The value of Rsc is given by
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𝑅𝑠𝑐 =
0.6
𝐼𝑙𝑖𝑚𝑖𝑡
POWER AUDIO AMPLIFIER IC LM380:
Features of LM380:
1. Internally fixed gain of 50 (34dB)
2. Output is automatically self centring to one half of the supply voltage.
3. Output is short circuit proof with internal thermal limiting.
4. Input stage allows the input to be ground referenced or ac coupled.
5. Wide supply voltage range (5 to 22V).
6. High peak current capability.
7. High impedence.
8. Low total harmonic distortion
9. Bandwidth of 100KHz at Pout = 2W & RL = 8Ω
Introduction
Small signal amplifier are essentially voltage amplifier that supply their loads with larger
amplifier signal voltage. On the other hand , large signal or power amplifier supply a large
signal current to current operated loads such as speakers & motors. In audio applications,
however, the amplifier called upon to deliver much higher current than that suppkied by
general purpose op-amps. This means that loads such as speakers & motors requiring
substantial currents cannot be driven directly by the output of general purpose opo-amps.
However there are two possible solutions,
To use discrete or monolithic power transistors called power boosters at the output of the
op-amp
To use specialized ICs designed as power amplifiers.
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LM380 circuit description:
It is connected of 4 stages,
(i) PNP emitter follower
(ii) Different amplifier
(iii) Common emitter
(iv) Emitter follower
(i) PNP Emitter follower:
The input stage is emitter follower composed of PNP transistors Q1 & Q2 which drives the
PNP Q3-Q4 differential pair.
The choice of PNP input transistors Q1 & Q2 allows the input to be referenced to ground
i.e., the input can be direct coupled to either the inverting & non-inverting terminals of the
amplifier.
(ii) Differential Amplifier:
The current in the PNP differential pair Q3-Q4 is established by Q7, R3 & +V.
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The current mirror formed by transistor Q7, Q8 & associated resistors then establishes the
collector current of Q9.
Transistor Q5 & Q6 constitute of collector loads for the PNP differential pair.
The output of the differential amplifier is taken at the junction of Q4 & Q6 transistors & is
applied as an input to the common emitter voltage gain.
(iii) Common Emitter:
Common Emitter amplifier stage is formed by transistor Q9 with D1, D2 & Q8 as a current
source load.
The capacitor C between the base & collector of Q9 provides internal compensation &
helps to establish the upper cutoff frequency of 100 KHz.
Since Q7 & Q8 form a current mirror, the current through D1 & D2 is approximately the
same as the current through R3.
D1 & D2 are temperature compensating diodes for transistors Q10 & Q11 in that D1 & D2
have the same characteristics as the base-emitter junctions of Q11. Therefore the current
through Q10 & (Q11-Q12) is approximately equal to the current through diodes D1 & D2.
(iv) (Output stage) - Emitter follower:
Emitter follower formed by NPN transistor Q10 & Q11. The combination of PNP transistor
Q11 & NPN transistor Q12 has the power capability of an NPN transistors but the
characteristics of a PNP transistor.
The negative dc feedback applied through R5 balances the differential amplifier so that the
dc output voltage is stabilized at +V/2;
To decouple the input stage from the supply voltage +V, by pass capacitor in order of micro
farad should be connected between the bypass terminal (pin 1) & ground (pin 7).
The overall internal gain of the amplifier is fixed at 50. However gain can be increased by
using positive feedback.
APPLICATIONS:
(i) Audio Power Amplifier:
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Amplifier requires very few external components because of the internal biasing,
compensation & fixed gain.
When the power amplifier is used in the non inverting configuration, the inverting terminal
may be either shorted to ground, connected to ground through resistors & capacitors.
Similarly when the power amplifier is used in the inverting mode, the non inverting
terminal may be either shorted to ground or returned to ground through resistor or capacitor.
Usually a capacitor is connected between the inverting terminal & ground if the input has a
high internal impedance.
As a precautionary measure, an RC combination should be used at the output terminal (pin
8) to eliminate 5-to-10 MHz oscillation.
C1 is coupling capacitor which couples the output of the amplifier to the 8 ohms loud
speaker which act as a load. The amplifier will amplify the Vin applied at the non-inverting
terminal.
Intercom system using LM 380:
When the switch is in Talk mode position, the master speaker acts as a microphone.
When the switch is in Listen position, the remote speaker acts as a microphone.
In either phone the overall gain of the circuit is the same depends on the turns of
transformer T.
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